Aromatic hydrocarbon compounds derived from petrochemical sources, benzene (C6H6), toluene (methylbenzene, C7H8), and xylenes (dimethylbenzenes, C8H10 isomers) may be used as starting materials for a wide range of consumer products. The xylenes include three isomers of dimethylbenzene, namely: 1,2-dimethylbenzene (ortho-xylene or o-xylene), 1,3-dimethylbenzene (meta-xylene or m-xylene), and 1,4-dimethylbenzene (para-xylene or p-xylene). The three isomers of xylene may be used in the synthesis of a number of useful products. For example, upon oxidation, the p-xylene isomer yields terephthalic acid, which may be used in the manufacture of polyester plastics and synthetic textile fibers (such as Dacron), films (such as Mylar), and resins (such as polyethylene terephthalate, used in making plastic bottles). The m-xylene isomer may be used in the manufacture of plasticizers, azo dyes, and wood preservers, for example. The o-xylene isomer may be used as a feedstock for phthalic anhydride production, which in turn may be used to make polyesters, alkyl resins, and PVC plasticizers. Therefore, the demand for xylenes remains strong as markets for polyester fibers and polyethylene terephthalate continue to demonstrate high growth rates.
Environmental regulations in several countries limit the amount of aromatics that can be blended into the gasoline pool. Most of the aromatics in gasoline originate from catalytic reforming of naphtha. Aromatic hydrocarbon compounds contained in a gasoline generally have higher octane values and are superior as a gasoline base, because of their high calorific values. Among them, toluene and aromatic hydrocarbon compounds, especially those having eight carbon atoms, have higher octane values and drivability levels, thus, it is desirable to increase the volume of C8 aromatic compounds in gasoline. Light reformate of the naphtha is blended into gasoline, because it has a high octane number and lower boiling point; however, environmental regulations exclude a substantial quantity of the heavy reformate in gasoline, thus making heavy reformates available for utilization elsewhere.
Typically, heavy reformate contains 90 weight (wt.) % to 95 wt. % C9 and 5 wt. % to 10 wt. % C10 aromatic compounds. Among the C9 components, trimethylbenzenes (TMBs) (50 wt. % to 60 wt. %) and methylethylbenzenes (MEBs) (30 wt. % to 40 wt. %) are the major constituents. One of the economically viable options is to convert the heavy aromatics in the heavy reformate into valuable products, such as xylenes. Demand is growing faster for xylene derivatives than for benzene derivatives. Therefore, a higher yield of xylenes at the expense of benzene yield is a favorable objective.
Heavy reformate can be subjected to transalkylation either alone or with C7 (toluene) for the production of xylenes (C8) and benzene (C6). Because many different compounds may be present in the heavy reformate, multiple parallel and consecutive reactions may take place. Transalkylation reactions for converting aromatic hydrocarbon compounds to compounds having a different number of carbon atoms may include the disproportionation reaction of toluene, i.e., two molecules of toluene react to form one molecule of benzene and one molecule of xylene (by transfer of a methyl group from one molecule of toluene to the other, a transalkylation reaction). Transalkylation reactions, however, are not limited to the disproportionation of toluene. Other methods of increasing xylene yields operate through inducing transalkylation by adding aromatic hydrocarbon compounds having nine or more carbon atoms into the starting materials, resulting in such reactions as the addition of one mole of toluene to one mole of a C9 aromatic hydrocarbon to produce two moles of xylene. These parallel and consecutive reaction methodologies may also be accompanied by multiple chemical equilibria, including isomerization of xylenes, TMBs and MEBs. The transalkylation and disproportionation reactions are equilibrium constrained, while the dealkylation reactions are kinetically controlled.
It is also known to separate isomers through molecular sieves formed by zeolites. Zeolites are generally hydrated aluminum and calcium (or sodium) silicates that can be made or selected with a controlled porosity for catalytic cracking in petroleum refineries, and may be natural or synthetic. The pores may form sites for catalytic reactions to occur, and may also form channels that are selective for the passage of certain isomers to the exclusion of others. Zeolites may serve as Brönsted acids for hydrogen ion exchange by washing with acids, or as Lewis acids by heating to eliminate water from the Brönsted sites. For example, the zeolite ZSM-5 (Na3Al3Si93O192.16H2O) has a pore size that results in the formation of channels of such size and shape that it forms a selective sieve for xylene isomers. The alkylation of toluene by methanol will form a mixture of all three xylene isomers. p-Xylene will pass through the channels in ZSM-5 due to its linear configuration, while o-xylene and m-xylene will not pass through the pores, although they may subsequently rearrange to p-xylene under the acidic conditions in the pores and then pass through the sieve. The catalytic activity of zeolites can also be increased by addition of a metal catalyst that activates hydrogen by breaking up molecular hydrogen to atomic hydrogen on the surface of the metal for forming intermediates in transalkylation reactions.
Regardless, these conventional means to produce xylenes by fractionation of reformate results in a xylene yield that is insufficient to meet the demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes. Furthermore, xylene isomer streams from catalytic reforming or other sources do not meet the demand as chemical intermediates. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but equates to only 20% to 25% of a typical C8 aromatics stream.